The Preparation of Combined Aluminium Oxide Coatings on Aluminium Substrate

M. ZEMANOVÁ, P. FELLNER, and M. CHOVANCOVÁ

Department of Inorganic Technology, Faculty of Chemical Technology,

Slovak Technical University, SK-812 31 Bratislava

Received 21 September 1995

T h e prepara t ion and propert ies of combined a luminium oxide coatings have been investigated. T h e coating has been prepared in two steps. First , a luminium subs t ra te was anodically oxidized. In t he next s tep the anodized film has been coated by the sol-gel method . T h e condit ions of forming coatings wi thout shrinkage were obta ined empirically. It was found out t h a t the film prepared by the sol-gel me thod has the same crystal s t ruc ture as the oxide film prepared by the anodic oxidation. T h e thickness, phase analysis, and surface micros t ructure of obta ined coatings were determined.»-

The coatings prepared by using of various methods can improve properties of functional coatings, for in­stance they can increase the corrosion resistance or im­prove mechanical properties [1]. In the present work, preparation and properties of aluminium oxide coat­ings formed by the combination of anodic oxidation and sol-gel process are discussed.

Anodic oxidation of metallic aluminium is an elec-trolytical process for production of porous alumina, which improves physical and chemical properties of coating and increases the field of application of alu­minium [2, 3]. The porous layer only marginally im­proves the corrosion resistance of the underlying alu­minium. Hence a method of sealing the pores is re­quired. There are several possibilities how to carry out the process. The hydrothermal method is done by immersing anodized samples into boiling water. The so-called cold sealing uses the reactions of the porous alumina layer with nickel and fluoride ions [4—6]. The sol-gel method is a new up to now unpublished way of sealing of porous anodized aluminium. This proce­dure provides an ability to reach new properties of the combined alumina coatings.

The sol-gel method is based on the transformation of the metal alkoxide solution into gel during hydroly­sis and polycondensation and continuing transforma­tion into glass and ceramics [7]. The outstanding ho­mogeneity of obtained materials, a relatively low reac­tion temperature (in our case lower than 400 °C), and ability of forming new materials and composites are the main advantages of glass and ceramics prepara­tion. The important sol-gel applications include thin glass or glass crystalline coatings. The layers on vari­ous substrates (glass, ceramics, metal) improve chemi­cal and physical resistance or change the optical prop­erties of the substrate. The most important advantage

of the sol-gel method is the ability to control the mi­crostructure of the deposited film precisely, i.e. the pore volume, pore size, and surface area [8].

The aim of this work was to find out the condi­tions for preparation of combined aluminium oxide coatings without shrinkage and cracks. Dependence of the thickness, structure, phase composition, and the conditions of preparation of the layer was investigated as well.

E X P E R I M E N T A L

The coating has been prepared gradually, in two steps: first, aluminium substrate was anodically oxi­dized, next, the film has been formed by sol-gel dip coating technique [9].

Anodic oxidation has been carried out in 20 mass % -sulfuric acid solution under following conditions: the bath temperature was kept in the range 18—20°C, the time of anodic oxidation was 15—40 min, and the cur­rent density was 1.5 A d m - 2 : Aluminium pieces (pu­rity 99.5 %) of dimensions 60 mm x 20 mm x 0.75 mm were used as substrate. Before anodic oxidation aluminium samples were degreased in an alkaline so­lution (Synalod 80, 50 g dm - 3 ) at 70°C. In the next step the samples were etched in sodium hydroxide so­lution (100 g dm - 3 ) at 50°C for 5 min, followed by the satin etching in the mixture of sodium hydroxide (90 g dm - 3 ) , sodium nitrate (109 g d m - 3 ) , and saccharose (1 g dm - 3 ) at 50°C, brightening in nitric acid solu­tion (400 g dm"3) at 50°C was realized. Between each stage of the pretreatment, samples were thoroughly rinsed.

Oxide film was formed by the anodic oxidation us­ing the procedure described above. The samples were rinsed, dried, and dipped into aluminium oxide sol

Chem. Papers 50 (2) 55—59 (1996) 55

M. ZEMANOVA, P. FELLNER, M. CHOVANCOVA

solution. This solution was prepared in two forms: alkoxide solution and colloid solution. Alkoxide solu­tion is based on partially hydrolyzed aluminium oxide solution obtained by mixing aluminium alkoxide with about 0.4—1 mol of water per 1 mol of aluminium alkoxide in the presence of sufficient amount of alco­hol. The final sol solution contained up to 10 mass % of alumina. The mixture is heated until clearing, which usually requires a temperature of 60 °C [8]. Then suf­ficient amount of additional water is added to bring the amount of substance of moles of water per 1 mol of alkoxide up to at least about 2 mol. A polymerized alumina glass can be prepared from the sol described above by heating up to the temperature of 400 °C [10]. The Yoldas process consists of hydrolyzing an aluminium alkoxide, Al(OR)3 (usually Al(OBus)3), in a large excess of water (r = 100—200, r being the mole ratio of water to Al(OBus)3) at 80—100 °C followed by peptization with a mineral acid (HNO3) to yield a sta­ble particulate sol. Gelation is generally achieved by concentration of the sol via evaporation [7].

In this work the Yoldas process was used. The sam­ples were immersed into the sol and withdrawn verti­cally with the speed of Uo = 11.7 cm min - 1 . Then the solvent was evaporated at temperature of 80 °C for 10 min. Dried samples were heated for 15 min at 400 °C in air [7].

The first series of experiments served like a proof of realization of combined alumina coatings on the alu­minium substrate. It was found out that the prepared combined films cracked when heated. Therefore in the next series of experiments the conditions for prepara­tion of combined alumina coatings which would nei­ther shrink nor crack were investigated. Different com­bination of the time of anodic oxidation, concentration of the sol solution, speed of withdrawing the samples from the sol solution, and the conditions of heating were tested.

The time of anodic oxidation ranged from 15 min to 40 min. The mixtures of Al(OBus)3 with water with the mole ratio r = 1:100, 1:150, 1:200 were used. Speeds of withdrawing were u0 = 1.6 cm min - 1 , 4.8 cm min - 1 , and 11.7 cm min - 1 , respectively. The con­ditions for drying were not changed. On the basis of DTA analysis of the samples it was found out that the optimal conditions for drying are 110°C and time of 30 min. Calcination of the samples was realized for 15 min at the temperature of 200 °C, 250 °C, 300 °C, 350 °C, and 400 °C, respectively. The various approaches of the heat treatment were used:

a) Gradual heating from ambient temperature to the goal temperature with the heating rate 10 °C min - 1 or 30°C min - 1 , calcination at the determined temperature for 15 min, cooling - spontaneous or con­trolled (10°C min - 1 or 50°C min - 1 cooling rate).

b) No preheating (the sample was placed into the oven heated to the chosen temperature), calcination at the determined temperature for 15 min, cooling -

spontaneous or controlled (10°C min - 1 or 50°C min"1

cooling rate). c) No preheating, calcination at the determined

temperature for 15 min (thermal shock realized only with giving into and taking out from the furnace), abrupt cooling to the ambient temperature.

The surfaces of the coating films were observed vi­sually and by scanning electron microscopy (SEM) using the model Jeol JSM 35 or Tesla BS 300. Mi-crostructure and phase content composition of alu­mina oxide coating were determined by the X-ray diffraction (XRD) using the model Dron UM1. Thick­ness of the surface film was determined gravimetrically and it was also measured on the cross-sections of the samples by X-ray microanalyzer Jeol JXA-840A.

RESULTS A N D DISCUSSION

The experimental results are summarized in Tables 1 and 2. Quality of prepared anodic film on aluminium substrate was evaluated visually. In Table 1, proper­ties of the films prepared by anodic oxidation of alu­minium are presented. In Table 2, properties of films prepared by combination of electrolytic anodizing and sol-gel procedure are given.

In the first series of experiments we were concen­trated upon the realization of combined alumina coat­ings on aluminium surface. Anodic oxidation has been carried out in sulfuric acid electrolyte (20 mass %) for 40 min (jA = 1.5 A d m - 2 ) . The other conditions of the treatment of the oxide layer are given in Table 1.

Combined alumina coatings were of good quality when the second film was prepared by the Yoldas pro­cess [8]. When we used the colloid method we did not succeed in preparation of compact film without cracks (see Fig. 1). Cracking of the surface film may be caused by different values of the coefficient of ther­mal expansion. The value of coefficient of thermal ex­pansion of aluminium is 23.8 x 1 0 - 6 o C - 1 , the coeffi­cient of thermal expansion of the anodized film is only about 20 % of that of aluminium and for the polycrys-talline alumina the coefficient of thermal expansion is 7 x l O - 6 ^ " 1 [2]. The problem of cracking was solved by changing of experimental conditions.

At first, the influence of chosen heat regimes on microstructure of anodically oxidized aluminium sub­strate was studied (see Table 1). The time of anodic oxidation as well as different heat regimes with vari­ous temperatures of calcination were altered. Varying time of anodic oxidation the thickness of the anodic oxide coating was changed.

We can conclude that heating did not influence coating microstructure. The heat shock (realized by giving the sample into furnace, its calcination and im­mediately taking it out from the furnace) caused the failure of surface coating. The spontaneous cooling regime from the temperature of calcination to the lab­oratory temperature has positively influenced the mi-

56 Chem. Papers 50(2)55—59 (1996)

COATINGS ON ALUMINIUM

Table 1. Dependence of the Quality of Alumina Coating Pre­pared by Anodic Oxidation of Aluminium Substrate on the Conditions of Preparation

t АО /min

40

15

25

30

0calc/°C

400

350

300

250

200

150

180

400

350

300

250

400 350 300

400 350

Heat treatment

H , -

— H, с

с н,—

— H, с

с H , —

— H, с

с H , —

— H, с

с н,—

— H, с

с н, —

— н, с

с н , -

— н, с

с н , -

— н, с

с н , -

— н, с

с н,—

— н, с

с н, —

— н, с

с с с с с с

Surface evaluation

S S S S

s s s s s s s s s s s s s s N N N N N N N N N N

S S N N S S N N N N N N N N N N

S S S

s s

T a b l e 2. Visual Evaluation (S - shrinkage, N - no shrinkage) of Combined Alumina Coatings in Dependence on Time of Anodic Oxidation t д о and Temperature of the Sol-Gel Film Calcination 0Calc

Í A o / m m 0calc/°C Surface evaluation

15

20

25

30

350 300 250 220

350 300 250 220

350 300 250 220

350 300 250 220

S N N N

S N N N

S S S N

S S S

s

*AO ~ time of anodic oxidation, 0caic - calcination temperature, H - heating, С - cooling, direct giving of the sample into the furnace and cooling at ambient temperature, S - shrinkage, N - good quality film without shrinkage and cracks.

crostructure of prepared samples. These microstruc-tures were evaluated visually and by scanning electron microscopy (SEM) (Fig. 2).

We found out no shrinkage after 40 min anodic

Fig . 1. Microstructure of combined aluminium oxide coatings prepared by anodic oxidation at the temperature 20 °C for 40 min at the current density 1.5 A d m - 2 and by the sol-gel method at the temperature of calcination 400°C (x 100 magn.).

oxidation at the temperature of calcination ranging between 150—200 °C followed by spontaneous cool­

erem. Papers 50 (2) 55—59 (1996) 57

M. ZEMANOVA, P. FELLNER, M. CHOVANCOVA

Fig . 2. Microstructure of the surface of combined alumina coatings for anodic oxidation time 15 min and calci­nation temperature 400 °C for the sol-gel process (x 5000 magn.).

ing. The film prepared by anodic oxidation for 40 min cracked if it was calcinated at 400 °C. Besides the con­ditions of calcination the influence of thickness of the anodized layer on the quality of the film was investi­gated as well. Thinner anodic film was prepared by

Fig . 4. Microstructure of surface coating without shrinkage for 20 min anodic oxidation and temperature of calcination for the sol-gel process of 250°C (x 5000 magn.).

15 min anodic oxidation followed by calcination at the temperature 400 °C and spontaneous cooling. No shrinkage of the alumina film has been observed.

According to DTA and TG curves (Fig. 3) water

500

e/°c

400

300

200

100

0

•-..

DTA

-

-

T ^ ^

4 TG

\

vi

i / - . x -/

-

.

1000

- 800

- 600

e

- 400

- 200

Fig . 3 . DTA, TG, and T record of AI2O3 sol prepared according to Yoldas.

58 Chem. Papers 50(2)55—59 (1996)

COATINGS ON ALUMINIUM

100

90

80

70

60

50

АО

30

20

10

0

i

y-f^+^y

----

I

1 1

_2

4

1 1

i

— • - i —

1

1 -

-

-

-

-

----

1 10 20 30 40

29/° 50 60 70

Fig . 5. X-Ray diffraction patterns of anodically oxidized (1) and sol-gel dip deposited (2) aluminium substrate.

and organic molecules of solvent have been removed in the temperature range 120—220 °C (part I—II). In the next series of experiments the anodized layer was covered by sol-gel film. However, when we used the thermal treatment described above, the surface film cracked. Therefore we investigated different conditions of alumina coatings on aluminium substrate which would yield a good quality surface film. The time of anodic oxidation was in the range of 15—40 min, the temperature of calcination was 250—400 °C. Accord­ing to the experimental results shown in Table 2, we can conclude that optimal time of anodic oxidation equal to 20 min and calcination temperature of 250 —300 °C is convenient for preparation of combined alumina coatings without shrinkage (Fig. 4). Statis­tic evaluation of obtained results confirmed that for preparation of good quality combined coatings the temperature of 250 °C is optimal.

XRD pattern of heat-treated anodized aluminium substrate is shown in Fig. 5. Mixture of transition alu­minas was identified, especially the peaks of transition Í-AI2O3. The peaks of aluminium metal are evident, too. Because of preferential orientation of planes only some peaks were recorded. It is evident that the crys­tal structure of alumina prepared by anodic oxidation is the same as that of alumina prepared by the sol-gel method.

The thickness of the films was determined by dif­ferent ways. The thickness of sol-gel alumina film was determined gravimetrically (at the density of alumina 3.99 kg m~3). It has been found that the procedure used in this work gives the film with the thickness of (0.20 ± 0.05) /mi. (Principle correctness of this re­sult has been confirmed by SEM investigation of cross-section of the film.)

Thus it can be stated that combined alumina coat­ings prepared by a two-step process involving the an­odic oxidation of aluminium substrate and the sol-

gel method have been developed. Surface film without shrinkage was formed at the time of anodic oxidation of 20 min and the temperature of calcination of 250 °C, followed by spontaneous cooling. The silver dull sur­face coating without shrinkage was obtained. Nowa­days we investigate the properties of this coating type. X-Ray analysis has shown that a thin polycrystalline film consists of a mixture of transition aluminas. The thickness of the sol-gel coating determined gravimet­rically is approximately 0.2 /xm.

R E F E R E N C E S

1. Antropov, L. I. and Lebedinskii , Yu. N., Kompozitsion-

nye elektrokhimicheskie pokrytiya i materiály. P . 5— 20. Tekhnika, Kiev, 1986.

2. Henley, V. F., Anodic Oxidation of Aluminium and its

Alloys. P . 1. Pe rgamon Press , Oxford, 1982. 3. Malinovský, M. and Roušar , L, Teoretické základy

pochodu anorganické technologie I* (Theoretical Fun­damenta ls of Processes of Inorganic Technology I.) P . 342—344. Nakladate ls tv í technické l i teratury, Alfa Publishers , P rague , Brat is lava, 1987.

4. Kalantary, M. R., Gabe , D. R., and Ross, D. H., J.

Appl. Electrochem. 22, 268 (1992). 5. Zemanova, M., Thesis. Slovak Technical University,

Bratislava, 1991.

6. Wernick, S., P inner , R., and Sheasby, P . G., The Sur­

face Treatment and Finishing of Aluminium and its Al­

loys. P . 320. ASM Internat ional , Finishing Publicat ion, Ohio, Middlesex, 1987.

7. Pach, L., Príprava oxidových materiálov sól-gél proce­

som. Slovak Technical University, Bratislava, 1988.

8. Brinker, C. J. and Scherer, G. W., Sol-Gel Science. P . 787, 68. Academic Press , Boston, 1990.

9. Makishima, A. and K u b o , H., J. Am. Ceram. Soc. 69,

C-127 (1986).

10. Yoldas, B . E., J. Mater. Sei. 12, 1203 (1977).

Translated by M. Zemanova

Chem. Papers 50 (2) 55—59 (1996) 59